1. Field of the Invention
The present invention relates to an X-ray diagnostic apparatus of the type having a storage luminescent screen for the latent storage of the respective X-ray image, having an X-ray exposure apparatus for producing an X-ray beam, a readout apparatus system with which, for the reproduction of an image, the storage luminescent screen is excited to luminescence by a scanning with a source of radiation, a detector for the acquisition of the light emitted by the storage luminescent screen, and an image reproduction system.
2. Description of the Prior Art
German OS 38 03 766, discloses an X-ray diagnostic apparatus of this type in which, as a radiation-sensitive converter, a storage luminophore is irradiated with X-rays so that unbonded electrons are produced therein that are stored in potential traps. With a readout system, the entire surface of this storage luminescent screen is scanned pixel-by-pixel by an additional source of radiation, for example a laser, so that the electrons stored in the traps are excited and can fall back into recombination centers, whereby the energy differential is radiated in the form of light quanta. In this way, it is possible to read out the stored X-ray image from the storage luminescent screen.
For the planiform scanning of the storage luminescent screen, a laser beam of the type known as a flying spot scanner is deflected by two mirrors in the vertical and horizontal directions, so that all pixels located on the storage luminescent screen can be scanned one after the other. The light emitted by the storage luminescent screen is acquired by two light-guiding plates, and is guided to two laterally attached line-shaped CCD light converters. The output signal of the detector is supplied to a conventional video chain for the reproduction of the X-ray image on a monitor.
In this context, barium fluoride-bromium chloride compounds activated with europium that are known from German OS 33 47 207 and that can be excited by visible light (photostimulation), can be used as storage luminophores. A Hexe2x80x94Ne laser, is usually employed which produces radiation having a wavelength of 633 nm.
From European Application 859 244, an X-ray diagnostic apparatus is known in which the plates of the storage luminescent screens are read out with CCD lines instead of a flying spot scanner. These lines are very sensitive to the exact maintenance and reproducibility of the distance between the layer to be read out and the line-shaped CCD detector. The larger the numerical aperture of the light collector (for example a fiber-optic plate), the more strongly distance variations are noticeable, as causing a worsening of the modulation transmission function (MTF).
In flying spot scanners, the storage luminophore film or foil can be held without difficulty on a flat base by vacuum suction, because the stimulation and reading out are carried out from the same side. In the case of a CCD line readout arrangement, this is generally not possible, due to the limitations imposed by the aperture. If a correspondingly large glass plate were supported only at the edges (which would allow readout and stimulation), the gravitational deformation would be approximately 50 xcexcm, however, a deflection of this sort leads to an undesirable reduction of the MTF.
An object of the present invention is to provide an X-ray diagnostic of the type described above that avoids, or at least significantly reduces, bendings [or deflections of the plate of the storage luminescent screen, without damaging the imaging layer containing the storage luminophore.
The object is inventively achieved in an X-ray diagnostic apparatus wherein, for scanning the storage luminophore, the screen is guided through access or an opening of a supporting structure having a rectangular cross-section, and is held in the scanning region by air bearings attached to the supporting structure on both sides of the opening, and wherein a line-shaped radiation source for stimulation radiation and a line-shaped light detector are mounted at the supporting structure. Due to the use of air bearings having small distance tolerances, no friction arises that could hinder the scanning process and damage the layer surface, so that the storage luminescent screen can be accurately held in the scanning region.
In an embodiment of the invention, the line-shaped radiation source can be attached to the supporting structure on the one side of the opening, and the line-shaped detector can be attached to the supporting structure on the other side of the opening.
It has proven advantageous for two air bearings to be provided on each of the sides at which the line-shaped radiation source and the line-shaped detector are respectively disposed.
The line-shaped source of radiation can be a photodiode array, and the line-shaped detector can be a CCD light converter.
In order to accommodate the weight of the storage luminescent screen with minimal shape deformation, the supporting structure can be realized as a frame that is resistant to bending.
The air bearings can inventively operate at 5.5 bar excess (i.e., above atmospheric) pressure, with gap widths of 6xc2x11 xcexcm.
Deflection of the storage luminescent screen is optimally prevented in an embodiment wherein the air bearings are arranged such that they have a distance from the edge of the storage luminescent screen that corresponds to one-fourth of the width of the storage luminescent screen, or so that they divide the width of the scanning region into three regions of equal size.